Device for observing waveform of microwave signal

ABSTRACT

Apparatus for displaying the waveform of a microwave signal which embodies means for producing an electron beam, deflecting said beam in synchronism with the microwave signal and permitting transmission of the beam only when it is subjected to a predetermined deflection to form a pulsed beam. Deflecting the pulsed beam by a second deflecting signal synchronized with the microwave signal while varying the phase difference by a third signal and then displaying the effect of deflection by the second signal on one axis of the display means and controlling the sweep of the other axis of the display means by said third signal.

United States Patent [191 Fujisawa Nov. 18, 1975 DEVICE FOR OBSERVING WAVEFORM OF MICROWAVE SIGNAL [75] Inventor: Kazuo Fujisawa, Ikeda, Japan [73] Assignee: Osaka University, Osaka, Japan [22] Filed: Mar. 14, 1974 [21] Appl. N0.: 451,031

[52] US. Cl. 315/9; 313/426; 313/432; 313/433; 315/378; 315/386 [51] Int. Cl. ..H01J 29/98; H01J 29/70; H01J 23/16 [58] Field of Search 315/9, 3, 25; 328/228, 328/121; 313/432, 426, 427, 421, 439

[56] References Cited UNITED STATES PATENTS 3,378,721 4/1968 Huston 315/30 Primary E.raminer-Robert Segal Attorney, Agent, or Firm-Eugene E. Geoffrey, Jr.

[57] ABSTRACT Apparatus for displaying the waveform of a microwave signal which embodies means for producing an electron beam, deflecting said beam in synchronism with the microwave signal and permitting transmission of the beam only when it is subjected to a predetermined deflection to form a pulsed beam. Deflecting the pulsed beam by a second deflecting signal synchronized with the microwave signal while varying the phase difference by a third signal and then displaying the effect of deflection by the second signal on one axis of the display means and controlling the sweep of the other axis of the display means by said third signal.

6 Claims, 7 Drawing Figures US. Patent Nov. 18, 1975 Sheet 2 of3 3,921,028

I 1 5 Vil 7' i A i V X 0 I,

DEVICE FOR OBSERVING WAVEFORM OF MICROWAVE SIGNAL This invention relates to an improved device for observing the waveform of a microwave signal and other extremely high frequency signals.

In order to observe the waveform of a microwave signal of high frequency, a sampling oscilloscope has been used. In this device, a voltage pulse, which is synchronized with the waveform to be observed and varies its phase bit by bit at every cycle, is produced and the waveform is sampled during the duration of this pulse and amplified for use in waveform display. As the duration of the voltage pulse must be short enough in com parison with the period of the waveform to be observed, the higher the frequency of the waveform becomes, the shorter the required duration of the pulse becomes. Thus, the prior techniques only enable observation of a waveform up to about 18 GHZ. In addition, there is little likelihood of this upper limit of observable frequency being raised materially in the near future with known devices.

This invention intends to raise the upper limit of the observable frequency much higher by utilizing an electron beam technique.

The device for observing the waveform of a microwave signal according to this invention comprises a source for producing an electron beam, first deflecting means for deflecting said electron beam under control of a first waveform which is identical to or synchronous with that of said microwave signal, shutter means for letting said electron beam pass therethrough only when predetermined deflection is effected by said first defleeting means, thereby forming a pulsed elctron beam, second deflecting means for deflecting said pulsed electron beam under control of a second waveform which is identical to or synchronous with that of said microwave signal, phase shifting means for varying the phase difference between said first and second waveforms under control of a third waveform, and display means for effecting a display of the deflection effected by said second deflecting means along the phenomenon axis, the time axis sweep of said display means being effected in synchronism with said third waveform.

Other features and operations of this invention will be described in more detail hereinunder with reference to the accompanying drawings. In the drawings:

FIG. 1 is a schematic perspective view, partly in block form, representing an embodiment of a device according to this invention,

FIG. 2 is a waveform diagram to aid in explaining the operation of the device of FIG. 1;

FIG. 3 is a schematic perspective view, partly in block form, representing another embodiment of this invention;

FIG. 4 is a schematic sectional side view, partly in block form, representing a further embodiment of this invention;

FIG. 5 is a display pattern diagram to aid in explaining the operation of the device of FIG. 4;

FIG. 6 is a schematic sectional side view, partly in block form, representing a still further embodiment of this invention; and

FIG. 7 is a schematic perspective view representing two embodiment of a specific component of the device of this invention.

Throughout the drawings, like reference numerals are used to denote corresponding structural components.

Referring now to FIG. 1, an evacuated envelope 1 contains an electron gun assembly 10 comprising a cathode electrode 11, a Wehnelt electrode 12, an anode electrode 13 and an electron lens assembly consisting of three electrodes l4, l5 and 16 at one end thereof and an electron lens assembly 30 consisting of three electrodes 31, 32 and 33, a drift tube 40, a conductor disc 50 having a pin hole 51 in the center, a pair of facing deflection plates and an electron lens assembly consisting of three electrodes 81, 82 and 83 in that order in front of the electron gun assembly 10. A phosphor screen is provided at the other end of the envelope 1. A pair of parrallel conductors 20 are located between the electron gun assembly 10 and the electron lens assembly 30 and a similar pair of parallel conductors 60 are located between the conductor disc 50 and the deflection plates 70, so that the electron beam emitted from the electron gun 10 passes the gaps between the both pairs of parallel conductors.

The parallel conductors 20 are connected to the output of the phase shifter 3 having a control terminal and an input terminal 2, the latter also being connected directly to the parallel conductors 60. The drift tube 40 is connected through a transformer 6 to one fixed contact 5b of a single-pole double-throw switch 5, while the other fixed contact 5a is connected to the control terminal of the phase shifter 3. A saw-tooth wave generator 4 is connected to the movable arm of the switch 5 and also to the deflection plates 70. The conductor disc 50 is grounded and the other electrodes are respectively connected as shown and fed with proper operation voltages from conventional voltage sources (not shown).

In operation, a microwave signal Pi to be displayed and observed, which has a waveform as shown in FIG. 2(2) for example, is applied to the input terminal 2. The input signal Pi is directly applied to the parallel conductors 61) and, at the same time, shifted in phase by 0 and applied to the parallel conductors 20 as a signal Pi as shown in FIG. 2(1).

The electron beam emitted from the electron gun 10 is deflected laterally by an electric field produced by the applied signal Pi between the parallel conductors 20 when it passes the gap between the conductors. When the voltage Vi of the signal Pi is zero, the electron beam is not deflected and passes straight through the pin-hole 51 of the disc 50, while, when the voltage Vi is not zero, it is deflected and collides against the disc 50 and is absorbed thereby. Consequently, the electron beam passes the pin-hole 51 of the disc 50 only at the time points (1),, (1) (153, (1) in FIG. 2(1) and enters the gap between the parallel conductors 60 in an intermittent pulselike form. In the gap of the parallel conductors 60, this electron beam pulse is deflected laterally by an electric field produced by the applied signal Pi as in the case of the parallel conductors 20.

As there is a phase difference of 0 between the signals Pi and Pi, the fragments of the electron beam which have passed the pin-hole 51 of the conductor disc 50 at the time points (b (1) (1) dn, respectively are deflected by angles which are proportional to the voltages A A A A respectively, as obviously illustrated in FIG. 2.

The saw-tooth wave generator 4 generates a sawtooth wave signal Pm of relatively low frequency and. when the movable arm of the switch 5 engages the fixed contact 5a. this saw-tooth wave signal Pm is applied to the control terminal of the phase shifter 3 to control the phase shift thereof. For example, if the phase shifter 3 is arranged so that the phase difference is varied by the signal Pm from zero to 211', the deflection effected by the parallel conductors 60 traces completely the waveform of the signal Pi. As the saw-tooth wave signal Pm is also applied to the deflection plates 70 and the electron beam is further deflected laterally by an electric field produced by the signal Pm between these plates 70 in the direction perpendicular to the deflection effected by the parallel conductors 60, the waveform of Pi as shown in FIG. 2(2) is displayed on the phosphor screen 90.

The abovementioned phase control can be effected by switching the movable arm of the switch to the fixed contact 5b to apply the saw-tooth wave signal Pm to the drift tube 40 instead of controlling the phase shifter 3. In this case, the phase shift angle 6 of the phase shifter 3 is maintained constant. However, as the potential of the drift tube 40 varies linearly in accordance with the slope of the saw-tooth Wave Pm it changes the speed of the electrons in the pulsed electron beam passing therethrough. Accordingly, the transit time of the electrons from the parallel contuctors to the parallel conductors 60 varies linearly and this results in the same phase shifting effect as that obtained when the switch 5 is in engagement with contact 50. If the transformer 6 is selected appropriately, the same ZIOtO-27T phase shift is obtained and the full wave of the signal Pi is traced by the deflection effected by the parallel conductors 60 and similarly displayed on the phosphor screen 90.

Due to the difference of magnification between the center and the periphery of the phosphor screen 90, the device of FIG. 1 may produce some distortion of the displayed image. The device of FIG. 3 overcomes this undesired distortion of the displayed image.

In the device of FIG. 3, an electron gun assembly 10, a pair of parallel conductors 20, an electron lens assembly 30, a drift tube 40, a pin-hole disc 50 and an electron lens assembly 80 are arranged in the neck portion of an evacuated envelope 1 in same manner as in the device of FIG. 1. Between the disc 50 and the electron lens 80, there is disposed a helical travelling wave guide 100 and two pairs of deflection conductors 110 arranged orthogonally with the tips of their inner ends facing each other. As in the device of FIG. 1, the parallel conductors 20 are connected through a phase shifter 3 to an input terminal 2 and a saw-tooth wave generator 4 is connected through a switch 5 to the control terminal of the phase shifter 3 and further through a transformer 6 to the drift tube 40. However. the saw-tooth wave generator 4 is coupled with a sine wave generator 7 and is synchronized by a sine wave signal generated thereby. The sine wave signal is also supplied to a twophase convertor 8 to be converted into a synchronous two-phase ac. voltage which is applied to the deflection conductors 110 to produce a rotating electric field therebetween.

The envelope 1 has an enlarged cylindrical head portion 150 and a phosphor display screen 151 is formed on the cylindrical side wall thereof. In the head portion 150, there are located a drift tube 120, a cylindrical electrode 130 having a flange 131, a disc electrode 132 4- facing the flange 131 and two facing annular electrodes 141 and 142 respectively surrounding the flange 131 and disc electrode 132.

In operation. a microwave signal Pi to be displayed is applied from the input terminal 2 through the phase shifter 3 to the parallel conductors 20 as in the device of FIG. 1 and. at the same time. is applied directly to the input terminal 101 of the helical guide 100. The electron beam emitted from the electron gun 10 and modulated to form pulses by the parallel conductors 20 and the pin-hole disc is axially velocity-modulated with the applied signal Pi in the helical guide and then radially deflected by the rotating electric field produced by the deflection conductors 110. The electron beam further passes the drift tube and the cylindrical electrode and enters a retarding electric field produced between the flange 131 and the disc electrode 132. In this retarding field, the beam is deflected normal to the tube axis and collides with the cylindrical phosphor screen 151 after being focused by the annular electrodes 141 and 142.

In this case, the deflection angle effected by the retarding field is inversely proportional to the speed of electrons. However, the speed of electrons is related to the voltage Vi of the signal Pi by means of velocity modulation in the helical guide 100 and, therefore, the longitudinal sweep on the phosphor screen 151 corresponds to the level of the signal Pi. Since the phase difference 6 between the signal Pi applied to the parallel conductors 20 and the signal Pi applied to the helical guide 100 is varied with the saw-tooth wave Pm and this phase shift is synchronized with rotation of the electric field produced by the deflection electrodes 110 as abovementioned, the waveform of the signal Pi is displayed on the cylindrical phosphor screen 151 at a period of rotation equal to the period of the sine wave produced by the sine wave generator 7. As in the case of FIG. 1, the drift tube 40 may be used for the phase shift by shifting the switch 5 to the other contact 51).

In the embodiments of FIGS. 1 and 3, the upper limit of the observable frequency is determined based only upon the ability of the parallel conductors or the helical travellings wave guide to provide the electron beam with an effective deflection or velocity modulation, and this is determined by the gap coeffieient of the modulation gap. For example, when the accelerating voltage of the electron beam is 10 killovolts and the axial effective length of the gap is 0.2 millimeter, the electron transit angle in the gap is about 21r/3 at 100 GHz of frequency of the wave to be observed, and the gap coefficient is about 0.83. Accordingly, satisfactory operation is obtained.

By designing the device of FIG. 3 such that the travelling speed of the wave. to be observed, in the helical travelling wave guide 100 becomes synchronous with the transit speed of the electrons, it enables more efficient velocity modulation than that afforded by the device of FIG. ll. In addition, due to a wide band characteristic of the helical travelling wave guide, it becomes possible to observe waveforms over a wide frequency range.

Referring next to FIG. 4, the arrangement and functions of components 10, 20, 40, 50, 60 and 80 in the evacuated envelope 1 are exactly same as those of the corresponding components of the device of FIG. 1. In this embodiment, however, the envelope 1 has no phosphor screen and an electron collector electrode is positioned instead at the end of the envelope 1 remote from the electron gun 10. The electrode 160 is grounded. Before the electrode 160, there are a pair of flat electrodes 171 and 172 arranged in a same plane and define a slit 170 therebetween. The electrodes 171 and 172 are grounded through resistors 173 and 174 respectively. A pair of deflection electrodes 181 and 182 are located between the parallel conductors 60 and the electron lens assembly 80 and grounded respectively through resistors 183 and 184. The electrodes 171 and 172 are also connected respectively through fixed contacts 191a and 19112 of a double-pole doublethrow switch 190 to a high gain amplifier 200' and the output of the amplifier 200 is connected through fixed contacts 192a and l92b of theswitch 190 respectively to the deflection electrodes 181 and 182 and also connected to the phenomenon axis input Y of a cathoderay tube oscilloscope1210. The output of a saw-tooth wave generator 4*is connected through a transformer 6 to the drift tube 40 and also connected to the time axis input X of the oscilloscope 210.

In operation, a microwave signal Pi to be displayed is applied to the parallel conductors and 60 and a sawtooth wave signal Pm is applied to the drift tube 40 in the same manner as in the operation'of the device of FIG. 1 and causes a deflection of the pulsed electron beam, which completely traces the waveform of the signal Pi, at the parallel conductors 60. In this embodiment, however, the electrons deflected upwardly are absorbed by the electrode 171 and those deflected downwardly are absorbed by the electrode 172, thereby inducing corresponding voltage drops across the resistors 173 and 174 respectively.

When the movable arm of the switch 190 engages contacts 191a and 192a, the voltage drop induced across the resistor 173 is applied to the amplifier 200 and the amplified voltage is applied to the deflection electrode 181 to deflect the electron beam downwardly. The such cancelling action continues until the electron beam is aligned with the tube axis and passes the slit 170 and, at that time, the output of the amplifier 200 appears as a dc. voltage proportional to the corresponding instantaneous value of the voltage Vi of the signal Pi. Therefore, when the output of the amplifier 200 is applied to the phenomenon axis input Y and the saw-tooth wave signal Pm is appliedto the time axis input X of the oscilloscope 210, a waveform .pattern as shown in FIG. 5(1) is displayed on the screens-This pattern is similar to the positive half of the waveform of the signal Pi.

When the movable arm of the switch 190 is turned to the contacts 191b and 192b, a waveform pattern as shown in FIG. 5(2), which is similar to the negative half of the waveform of Pi, is displayed, for the same reason set forth above.

In this embodiment, the electron trajectory is deflected once by the microwave signal Pi on the parallel Conductors 60 but it is always forced back to the original straight trajectory by the low frequency deflection effected by the deflection plates 181 and 182. Therefore, the displayed pattern is not materially affected by distortion of the electron lens system and the low frequency deflection voltage is accurately proportioned to the wave-height of sampled portion of the wave to be displayed.

According to the device of this embodiment of the invention, the waveform display can be effected by an arbitrary separate display device. Although the description involves the use of a cathode-ray tube oscilloscope 6 210, a mechanical recording device such as XY- recorder can be used for this purpose if the selected frequency of the saw-tooth wave signal Pm is low enough.

FIG. 6 represents a variation of the device of FIG. 4. Arrangement and the operation of those components in this device. such as electron gun assembly 10, parallel conductors 20, drift tube 40, conductor disc 50, electron lens assembly 80, electrodes 171 and 172 and electron collector electrode 180, are substantially same as those in the device of FIG. 4. The output of a sawtooth wave generator 4 is also connected to the drift tube 40 and the time axis input X of a cathode-ray tube oscilloscope 210. In this device, however, a helical travelling wave guide 100 which is similar to that of the device of FIG. 3 is located between the disc electrode 50 and the electron lens 80, instead of the parallel conductors and the deflection electrodes 181 and 182 of the device of FIG. 4. Moreover, the electrodes 171 and 172 are respectively connected to separate high gain amplifiers 201 and 202 and the outputs of these amplifiers are connected through fixed contacts 220a and 220b of a single-pole double-throw switch 220 to the phenomenon axis input Y of the oscilloscope 210 and also to the control terminal 231 of a variable voltage generator 230 whose output is connected to the intermediate electrode 82 of the electron lens assembly 80.

In operation, a microwave signal Pi to be displayed is applied to the parallel conductors 20 and the helical guide and a saw-tooth wave signal Pm is applied to the drift tube 40 in the same manner as in the operation of the device of FIG. 3 and causes a velocity modulation of the pulsed electron beam which completely traces the waveform of the signal Pi, as described in conjunction with FIG. 3. Due to its own eccentricity and the eccentricity of the electron lens, the electron beam is deflected substantially by the electron lens 80 and collides with the electrode 171 or 172, as shown in the drawing. The magnitude of deflection imparted by the electron lens 80 to the electron beam relates to the electron speed and the negative potential applied to the electron lens electrode 82 by the variable voltage generator 230.

When the negative potential of the electrode 82 is at a reference level and the instantaneous voltage of the signal Pi is zero at the helical guide 100, the electron beam passes through the slit between the electrodes 171 and 172 and is absorbed by the collector electrode 180. If the instantaneous voltage of the signal Pi is positive, however, the electrons are accelerated to reduce the deflection angle and collide with the electrode 171. On the contrary, if the voltage of the signal Pi is negative, the electrons are decelerated to increase the deflection angle and collide with the electrode 172. Consequently, voltage drops corresponding to the electron flow are caused across the resistors 173 and 174 and amplified respectively by the amplifiers 201 and 212. When the movable arm of the switch 220 is in engagement with the contact 220a, the variable voltage generator 230 is controlled by the output of the amplifier 201 and drives the potential of the lens electrode 82 more negative. Therefore, the electrons are decelerated to increase the deflection angle and, when the T electrons having collided against the electrode 171 tend to pass the slit 170, a balance is attained. The output voltage of the amplifier 201 at this time is proportional to the positive voltage of the sampled portion of the signal Pi. Accordingly, for the same reason as in the device of FIG. 4, the oscilloscope 210 displays a waveform pattern as shown in FIG. (1) which corresponds to the positive half of the waveform of the signal Pi. Similarly, a display pattern as shown in FIG. 5(2) is obtained when the movable arm of the switch 220 is in engagement with contact 22011.

As the signal Pi acts on the electron beam by the helical guide 100 for a longer time than by the parallel conductors 60 of the device of FIG. 4, the device of FIG. 6 exhibits a higher efficiency than the former device. In addition, due to the wide band characteristic of the helical guide, it has a wider observable frequency range.

As above described, waveform sampling can be effected within a time interval which is extremely shorter than that available in the prior sampling devices by utilizing pulse modulation of electron beam, according to this invention, and it becomes possible to observe a waveform of much higher frequency. Moreover, another great advantage of this invention is that the sampling pulse is always synchronous with the wave to be observed and, therefore, it is also possible to observe a waveform of repeated pulse oscillation including jittering which has been impossible to be observed by the prior sampling Oscilloscopes.

It should be noted that the above description has been made only in conjunction with certain embodiments and various modifications and changes may be applied without departing from the scope of this invention as defined in the appended claims. For example, the parallel conductors and 60 were shown in the drawings as a pair of parallel straight conductors but they can take other shapes as shown in FIG. 7. FIG. 7( 1 represents a pair of conductors each in the form of a somewhat sinusoidal configuration and disposed in spaced parallel planes. FIG. 7(2) represents a helical conductor surrounded by a cylindrical conductor with both conductors being coaxial or parallel. These parallel conductors are well known by those skilled in the art and are not the subject of this invention.

I claim:

1. A device for observing the waveform of a microwave signal comprising a source of said microwave signal, means producing a continuous electron beam, first beam deflecting means, means producing a first waveform signal synchronous with the microwave signal to be observed, means connecting the last said means with said first deflecting means for deflecting said beam (in one plane), shutter means positioned forwardly of said first deflecting means for permitting said beam to pass when said deflection is within predetermined limits thereby producing a pulsed electron beam, second beam deflecting means following said shutter means for deflecting said beam (in said plane), a second means producing a second waveform signal in the form of and synchronous with said microwave signal, means producing a third waveform signal. phase shifting means for varying the phase between said first and second waveform signals with respect to said electron beam and under control of said third waveform signal. means applying said second waveform signal to said second beam deflecting means, display means in the path of and responsive to said beam for displaying the movement of said beam wherein the deflection produced by said second deflecting means occurs along the phenomenon axis and means for sweeping said beam in synchronism with said third waveform signal.

2. The device according to claim 1, wherein said first deflecting means comprises a pair of parallel conductors to which said microwave signal is applied, and said shutter means comprises a flat electrode having an orifice therein and located in front of said parallel conductors.

3. The device according to claim 1, wherein said phase shifting means comprises a tubular electrode positioned between said first deflecting means and said shutter means with said electron beam passing therethrough and said third waveform is applied to said tubular electrode.

4. The device according to claim 1, wherein said second deflecting means comprises a helical travelling wave conductor to which said second waveform is applied and means for deflecting said pulsed electron beam in a second plane angularly disposed relative to the first said plane, and said display means comprises a cylindrical display screen and means of applying a rotational field to said electron beam in synchronism with said third waveform thereby effecting a circumferential time axis sweep on said screen.

5. The device according to claim 1, wherein said display means comprises means for producing a voltage corresponding to the deflection effected by said second deflecting means, means interconnected with the last said voltage for cancelling said deflection, and a display device having a phenomenon axis sweep controlled by said voltage and time axis sweep controlled by said third waveform.

6. The device according to claim 5, wherein said voltage producing means comprises at least one electrode for collecting said electron beam encountering a predetermined deflection and a resistor connected to said electrode, and said deflection cancelling means comprises at least one deflection electrode for deflecting said electron beam opposite to the deflection effected by said second deflecting means under control of a voltage corresponding to a voltage produced across 

1. A device for observing the waveform of a microwave signal comprising a source of said microwave signal, means producing a continuous electron beam, first beam deflecting means, means producing a first waveform signal synchronous with the microwave signal to be observed, means connecting the last said means with said first deflecting means for deflecting said beam (in one plane), shutter means positioned forwardly of said first deflecting means for permitting said beam to pass when said deflection is within predetermined limits thereby producing a pulsed electron beam, second beam deflecting means following said shutter means for deflecting said beam (in said plane), a second means producing a second waveform signal in the form of and synchronous with said microwave signal, means producing a third waveform signal, phase shifting means for varying the phase between said first and second waveform signals with respect to said electron beam and under control of said third waveform signal, means applying said second waveform signal to said second beam deflecting means, display means in the path of and responsive to said beam for displaying the movement of said beam wherein the deflection produced by said second deflecting means occurs along the phenomenon axis and means for sweeping said beam in synchronism with said third waveform signal.
 2. The device according to claim 1, wherein said first deflecting means comprises a pair of parallel conductors to which said microwave signal is applied, and said shutter means comprises a flat electrode having an orifice therein and located in front of said parallel conductors.
 3. The device according to claim 1, wherein said phase shifting means comprises a tubular electrode positioned between said first deflecting means and said shutter means with said electron beam passing therethrough and said third waveform is applied to said tubular electrode.
 4. The device according to claim 1, wherein said second deflecting means comprises a helical travelling wave conductor to which said second waveform is applied and means for deflecting said pulsed electron beam in a second plane angularly disposed relative to the first said plane, and said display means comprises a cylindrical display screen and means of applying a rotational field to said electron beam in synchronism with said third waveform thereby effecting a circumferential time axis sweep on said screen.
 5. The device according to claim 1, wherein said display means comprises means for producing a voltage corresponding to the deflection effected by said second deflecting means, means interconnected with the last said voltage for cancelling said deflection, and a display device having a phenomenon axis sweep controlled by said voltage and time axis sweep controlled by said third waveform.
 6. The device according to claim 5, wherein said voltage producing means comprises at least one electrode for collecting said electron beam encountering a predetermined deflection and a resistor connected to said electrode, and said deflection cancelling means comprises at least one deflection electrode for deflecting said electron beam opposite to the deflection effected by said second deflecting means under control of a voltage corresponding to a voltage produced across said resistor. 